Thermionic cathode comprising mixture of barium oxide, calcium oxide and samarium oxide

ABSTRACT

A cathode is disclosed utilizing an emission material of barium oxide, calcium oxide and samarium oxide.

Unite States Meat Cronin Nov. 25, 1975 [54] THERMIONIC CATHODECOMPRISING 1,921,066 8/1933 Bedford 252/521 MIXTURE OF BARIUM OXIDE,CALCIUM 2,744,073 5/1956 Todd et a1. 1 1 252/521 2,813,807 11/1957 Levi117/224 OXIDE AND SAMARIUM OXIDE 3,076,916 2/1963 K0ppius.... 1,17/224[75] Inventor: Leo J. Cronin, Watsonville, Calif. 3, 8,08 H 964 K ppusnu 1 7/223 3,155,864 11/1964 Coppola... 117/224 Asslgneer Spectra-Mat,Watsonvflle, 3,719,856 3/1973 Koppius.... 313/346 R Calif. I

[22] Filed: 1972 Primary Examiner-Charles E. Van Horn 21 APPL 223 539Assistant ExaminerMichael W. Ball Attorney, Agent, or FirmLimbach,Limbach & Sutton [52] US. Cl 428/312; 313/311; 313/346 R; 252/521;428/472 [51] Int. Cl H01j l/l4;1-101j 12/06 [58] 'Field of Search117/224, 223; 252/521; [57] ABSTRACT 313/346 311 A cathode is disclosedutilizing an emission material of barium oxide, calcium oxide andsamarium oxide. [56] References Cited UNITED STATES PATENTS 8 Claims, 4Drawing Figures 1,794,298 2/1931 Just 117/220 US. Patent Nov. 25, 1975 01500 5000 4500 0000 L|FE(HOURS) 050 900 950 BRIGHTNESS TEMPERATURE ("0)I000 2000 5000 LIFE (H0003) 0 0 00 0 n v 0 OOOOOOTO 0 0 0 0 4 2 0000 W.m 00 6 w98 FEGA THERMIONIC CATI-IODE COMPRISING MIXTURE OF BARIUM OXIDE,CALCIUM OXIDE AND SAMARIUM OXIDE The present invention is directed ingeneral to cathodes and. more particularly, to thermionic dispensercathodes.

Over the years various different cathode structures have been proposedwith selected different materials to achieve high current densities, lowevaporation and long life. Cathodes have been made in many forms. In oneform, the emission material is sprayed or painted on the surface of asupport member, typically nickel, tungsten or molybdenum. Ribbons, wiresor screens have been added to the surface to aid in holding the emissionmaterial on the surface. Additionally, porous body structures have beenutilized to slowly dispense the emission material through the pores tothe emission surface. U.S. Pat. No. 2,543,728 to Lemmens et al.describes the use of a cavity to contain the emission material and aporous body portion providing communication between the cavity and theelectron emission surface for migration of the emission material. U.S.Pat. No. 2,700,000 to Levi et a1. describes a cathode wherein theemission material is melted into the pores of the porous-sintered body.

Barium oxide (BaO) has been the principal preferred emission materialfor many cathodes. However, barium oxide is extremely hygroscopic and innormal atmosphere readily converts to barium hydroxide. Consequently,many cathodes include barium oxide in a composition with anothermaterial whereby the barium oxide and/or free barium can be released fordesired electron emission. Barium carbonate has frequently been used asone composition wherein the carbonate will convert to barium oxide atelevated temperatures.

In thermionic dispenser cathodes operating at higher temperatures andutilizing refractory metal bodies such as tungsten, molybdenum and thelike as the poroussintered main body portion of the cathode, bariumcarbonate reacts with tungsten in a manner unproductive of free barium.Consequently, a number of alkaline earth metal compositions aresuggested wherein barium oxide will be held in a stable form. Materialssuggested in U.S. Pat. No. 2,700,118 to Hughes et al. are alkaline earthmetal silicates, aluminates, thorates, berylliates, and borates. Ofthese materials, the barium aluminates were preferred and have been usedmost extensively.

Since it was known, as in Therminoic Emission from the BaO-CaO System byL. E. Grey, NATURE, Volume 165, pp. 773-774, May 13, 1950, that enhancedemission can be achieved by the substitution of calcium oxide for partof the barium oxide, barium calcium aluminate dispenser cathodes havebeen used. U.S. Pat. No. 3,076,916 to Koppius describes one suchdispenser cathode having a porous-sintered tungsten body with animpregnate material formed of barium oxide, calcium oxide and aluminumoxide in the mole ratio of 4:1:1, respectively. U.S. Pat No. 3,201,639describes a similar thermionic dispenser cathode wherein the impregnatematerial is barium oxide, calcium oxide and aluminum oxide in the moleratio of 5:312, respectively.

U.S. Pat. 1,794,298 to Just describes a thermionic cathode using amixture of an oxide of the alkaline earth group of metals with a verysmall addition of one of the oxides of the rare earth group of metalsespecially thorium oxide, praseodymium oxide and samarium oxide. TheJust patent states that amount to which the rare earth oxide should bepresent for optimum results is determined to be approximately 0.1% ofthe amount of alkaline earth oxide or similar ingredient.

Broadly stated, the present invention is directed to a cathode having ametallic body and an emission material comprising a mixture of bariumoxide, calcium oxide and samarium oxide and wherein the samarium oxideis present in more than trace amounts.

It has been found that cathodes having the preferred compositions of thepresent invention produce enhanced emission over cathodes similarlyconstructed but having the most popular prior art emission material madeup of barium oxide, calcium oxide and aluminum oxide. The cathode of thepresent invention can produce the same emission as the most popularprior art cathode but up to about 50-75C cooler. This permits a lowerheater wire temperature and faster tube warmup time to achieve emissionand requires less heater power.

It has also been discovered that the cathode of the present inventionproduces up to 10 times less barium evaporation.

Additionally, the cathode of the present invention can be activated at atemperature approximately C lower than the activation temperature of thepopular prior art cathodes.

Other objects and advantages of this invention will become apparent whenreading the following description and referring to the accompanyingdrawing in which similar characters of reference represent correspondingparts in each of the several views.

In the drawings:

FIG. 1 is a side sectional view of a cathode inaccordance with oneembodiment of the present invention.

FIG. 2 is a graph of emission in milliamps plotted versus life for acathode in accordance with the present invention as contrasted with twocathodes in accordance with the prior art.

' FIG. 3 is a graph of current in milliamps plotted versus cathodetemperature for two cathodes in accordance with the present invention ascontrasted with two cathodes in accordance with the prior art.

FIG. 4 is a graph of temperature plotted versus life for a cathode inaccordance with the present invention as contrasted with a cathode inaccordance with the prior art.

Referring now to FIG. 1 there is shown one embodiment of the presentinvention. In this embodiment of the present invention the cathode 10includes a porous metallic body 1 l, of a refractory material, such astungsten, provided with an emission material 12 substantially uniformlydispersed throughout the pores of the body for migration through thepores to an emission surface 13. The emission material 12 can be placedin the pores by mixing, pressing and sintering material 12 with metalparticles such as tungsten particles when the body is of tungsten.Alternately, the cathode 10 can be formed by first forming the poroussintered body 11 by pressing and sintering tungsten particles and thenmelting the emission material into the pores of the body. An example ofsuch a cathode is described below.

The body 11 is supported for use in a vacuum tube by means of a supportcylinder 14, such as molybdenum. A heater filament 15 such as a metalwire wound in a spiral is positioned within the cylinder 14 behind thebody 11 for heating the body 11 and emission material 12 for thermionicemission of electrons from the emission surface 13 when the cathode ispositioned within a vacuum envelope (not shown). Cathodes in accordancewith this invention are suitable for use in most vacuum tubes or gastubes. The cathode can be used as a cold cathode" wherein the heater 14is utilized only to initiate emission or dispensed with entirely andemission derived in other ways such as by electron bombardment toproduce secondary emission.

The heater filament is placed in location within the cylinder 14 andpotted in placed with an electrical insulating material 16 such asalumina ceramic.

The emission material 12 is a mixture of barium oxide, samarium oxideand another alkaline earth metal oxide, other than barium oxide,preferably calcium oxide. I prefer as an emission material onecomprising barium oxide, calcium oxide and samarium oxide, Specificcompositions of the emission material in accordance with the presentinvention are set forth in greater detail below and with reference tothe method of formation of these materials.

However, the amount of samarium oxide must be greater than traceamounts. For a cathode in accordance with the present invention the molpercent of samarium oxide should not be less than about 12% and thesamarium oxide should not be less than about by weight. The preferredrange for the samarium is 16 to 20 mol percent or 34 to 43% by weight.

I have discovered that a cathode made generally in accordance with thepresent invention produces greater emission and has less bariumevaporation than the conventional barium oxide, calcium oxide, andaluminum oxide cathode. Also, an identical cathode body impregnated withan emission material of barium oxide, calcium oxide and samarium oxideproduces satisfactory emission at a lower temperature than the popularbarium oxide, calcium oxide and aluminum oxide type cathodes.

' I have discovered that cathodes with emission materials formed of 4moles of barium oxide, l mole of calcium oxide and 1 mole of samariumoxide as well as 5 moles of barium oxide, 3 moles of calcium oxide and 2moles of samarium oxide produce greater emission and less bariumevaporation than similar cathodes with emission materials of bariumoxide, calcium oxide and aluminum oxide in these same mole ratios.

FIGS. 2 and 3 show graphs of emission versus life and emission versusoperating temperature for cathodes in accordance with the presentinvention designated S as contrasted with the more popular barium oxide,calcium oxide and aluminum oxide cathodes designated type B. Thesubscript 1 (i.e., B-1 and S-l) refer to cathodes where the respectivematerials are in the mole ratio of 4:1:1, and the subscript 2 (i.e., B-2and 8-2) refer to cathodes where the respective materials are in themole ratio of 5:3:2.

FIG. 4 shows a graph of the apparent cathode temperature measured withan optical pyrometer through the glass of a test diode versus cathodelife. A decrease in apparent temperature is a measure of the amount ofbarium evaporated from the cathode onto the glass bulb. FIG. 4 shows alower evaporation rate for cathodes of the present invention contrastedwith popular prior art cathodes. In FIG. 4 curves are shown only for B-1and S-2; a B-2 cathode has 3 or 4 times the evaporation rate of a B-1and the curve therefore would start higher at the left and decrease witha greater slope to the right; and 5-] has a slightly lower evaporationrate than 5-2.

4 While it is believed that the invention has been described thus far insufficient detail to enable one skilled in the art to manufacturecathodes in accordance with the present invention, a detailed operativeexample ofa 5 cathode of the type illustrated in FIG. 1 will be given.

First of all, the body 11 is formed by pressing tungsten powder,approximately 5 microns average diameter, to form an ingot which isheated in hydrogen at about 2350C for approximately 20 minutes to form aporous-sintered body having a density in the range of 7884%. Next, theporous body is impregnated with a filler material such as a metal orplastic and the filled ingot machined to form individual buttons orbodies having the dimensions of the desired cathodes. In the case ofmetal, such as copper, the filler material can be melted into the pores,while in the case ofa plastic such as methyl methacrylate, the porousbody can be impregnated with the liquid plastic which then sets up andhardens in the pores. After machining, the filler material is removed byheating to elevated temperature.

For the emobidment having an emission material of barium oxide, calciumoxide and samarium oxide in the mole ratio of 4:1:1, 4 moles bariumcarbonate, 1 mole calcium carbonate and 1 mole samarium oxide are heatedto a temperature of about 1 100C in an air atmosphere. At this elevatedtemperature, the carbonates convert to the respective oxides and theseoxides form compounds. The resultant material is allowed to cool.

For impregnating the emission material into the cathode body the cooled,hardened mass is pulverized to form a powder and heated in a hydrogenatmosphere to a temperature of about l550C. and brought to contact withthe previously machined cathode body whereby the emission material willmigrate into the pores of the body by capillary attraction. Excessemission material is removed from the emission surface.

The heater may be potted" (before impregnation) within the cylindricalsupport behind the cathode body by surrounding the heater with powderedalumina ceramic and heating the assembly to a temperature of aboutl900C. to sinter the alumina.

The completed cathode can be mounted and activiated in an electrondischarge device by heating the cathode to 1100C. while exhausting thedevice. This activation temperature is at least 50100C. below theactivation temperature required for the popular barium oxide, calciumoxide and aluminum oxide (513:2 mole ratio) cathodes.

While calcium oxide is preferred as the other alkaline earth metal to beutilized with barium oxide and samarium oxide, other alkaline earthmetal oxides such as strontium oxide and magnesium oxide can be used.

The emission material can be applied to other surfaces such as byevaporation onto a metal surface from which emission is to be drawn.

Additionally the cathode can be made in the form of a directly heatedcathode body as described in US. Pat. No. 3,514,66l to R. T. Reavesassigned to the assignee of this invention.

What is claimed is:

1. A cathode comprising:

a metallic body structure of a high melting point metal and an emissionmaterial supported by said body structure,

said emission material comprising a mixture of barium oxide, calciumoxide and samarium oxide with the amount of samarium oxide present inthe mixture in excess of about 12 mol percent.

2. A dispenser cathode comprising:

a refractory metal body having an electron emission surface and a porousportion for migration of an emission material to said surface andemission material supported by and in contact with said body formigration through said porous portion to said surface.

said emission material comprising a mixture of barium oxide. samariumoxide and a second alkaline earth metal oxide other than barium oxidewith the amount of samarium oxide present in the mixture in excess ofabout 12 mol percent.

3. The dispenser cathode in accordance with claim 2 wherein said secondalkaline earth metal oxide is calcium oxide.

4. The dispenser cathode in accordance with claim 3 wherein said bariumoxide, calcium oxide and samarium oxide are in the mole ratio of about411:1, respectively.

5. The dispenser cathode in accordance with claim 3 wherein said bariumoxide. calcium oxide and samarium oxide are in the mole ratio of about513:2 respectively.

6. A dispenser cathode comprising:

a porous high-density tungsten body with interconnecting porescommunicating with an electron emission surface and an emission materiallocated within said pores for migration through said pores to saidsurface. said emission material comprising a mixture of barium oxide,samarium oxide and a second alkaline earth metal oxide other than bariumoxide with the amount of samarium oxide present in the mixture in excessof about l2 mol percent.

7. The dispenser cathode in accordance with claim 6 wherein said secondalkaline earth metal oxide is calcium oxide.

8. The dispenser cathode in accordance with claim 7 wherein said bariumoxide. calcium oxide and samarium oxide are in the mole ratio of about4:111, respectively.

1. A CATHODE COMPRISING: A METALLIC BODY STRUCTURE OF A HIGH MELTING POINT METAL AND AN EMISSION SUPPORTED BY SAID BODY STRUCTURE, SAID EMISSION MATERIAL COMPRISING A MIXTURE OF BARIUM OXIDE, CALCIUM AND SAMARIUM OXIDE WITH THE AMOUNT OF SAMARIUM OXIDE PRESENT IN THE MIXTURE IN EXCESS OF ABOUT 12 MOL PERCENT.
 2. A dispenser cathode comprising: a refractory metal body having an electron emission surface and a porous portion for migration of an emission material to said surface and emission material supported by and in contact with said body for migration through said porous portion to said surface, said emission material comprising a mixture of barium oxide, samarium oxide and a second alkaline earth metal oxide other than barium oxide with the amount of samarium oxide present in the mixture in excess of about 12 mol percent.
 3. The dispenser cathode in accordance with claim 2 wherein said second alkaline earth metal oxide is calcium oxide.
 4. The dispenser cathode in accordance with claim 3 wherein said barium oxide, calcium oxide and samarium oxide are in the mole ratio of about 4:1:1, respectively.
 5. The dispenser cathode in accordance with claim 3 wherein said barium oxide, calcium oxide and samarium oxide are in the mole ratio of about 5:3:2 respectively.
 6. A dispenser cathode comprising: a porous high-density tungsten body with interconnecting pores communicating With an electron emission surface and an emission material located within said pores for migration through said pores to said surface, said emission material comprising a mixture of barium oxide, samarium oxide and a second alkaline earth metal oxide other than barium oxide with the amount of samarium oxide present in the mixture in excess of about 12 mol percent.
 7. The dispenser cathode in accordance with claim 6 wherein said second alkaline earth metal oxide is calcium oxide.
 8. The dispenser cathode in accordance with claim 7 wherein said barium oxide, calcium oxide and samarium oxide are in the mole ratio of about 4:1:1, respectively. 